The present invention is directed to filtration systems and, more particularly, to a system and method for removing contaminants from groundwater associated with underground natural gas wells, mining operations, and other environments.
It is often necessary to remove water from underground geological formations in order to release natural gas associated with the underground formations. Oftentimes, the formations are more than 1,000 feet below the surface of the earth. A typical formation can comprise several separate layers of the liquid and gas or can comprise a single large reservoir. A bore hole is drilled into the earth and passes through the different layers of the formation until the target layer is reached. The location and depth of the bore hole is carefully controlled because of the great expense associated with drilling the bore hole. In order to prevent collapse of the bore hole after drilling, it is usually lined with a casing along its entire length. The casing also helps to control reservoir pressure and protect surface water from contamination. The casing is cemented in place and sealed at the ground surface by a wellhead.
One or more pipes or tubes extend into the bore hole from the wellhead. One of the tubes is typically used to carry liquid to the surface. The internal pressure of many geological formations is often insufficient to naturally raise commercial quantities of the natural gas from the formation through the bore hole or does so at an inadequate flow rate. Oftentimes, a large volume of liquid is present in the underground formation and must be removed on a continuous basis in order to recover natural gas from the formation. An artificial lift system is used in conjunction with the tube(s) to remove the liquid from the underground formation. Currently, many different types of artificial lift systems are available to lift the liquid from the formation, the most common of which are progressive cavity pumps, beam pumps and subsurface gas lift (SSGL) systems.
No matter what artificial lift system is used, water retrieved from underground coal formations often contains contaminants such as iron and inorganic and organic sulfur compounds, manganese, sodium, barium, arsenic, and other trace metals, and coal fines. Some constituents indigenous to groundwater associated with underground coal seams, such as iron and manganese, are pH dependent, while other constituents, such as sulfur, are more oxygen dependent. These constituents typically exist in soluble forms in the groundwater. When the groundwater is ejected or pumped to the surface and exposed to air, iron, sulfur and manganese are oxidized, resulting in the deposition of insoluble forms and precipitate on contact surfaces causing discoloration. The precipitates may also impart foul taste and odor to the water.
Federal and state regulations dictate minimum water quality standards that must be met before water from underground formations, mines or other underground structures can be discharged into the environment. When such standards are not met, the environment may be adversely affected and gas production or other operations may be halted. Due to the excessive amounts of contaminants in the discharge water, many natural gas producers are experiencing difficulties in obtaining discharge permits from state agencies.
Prior attempts to filter the contaminated water have included activated carbon filters, capacitive deionization systems, and the like. The filters can become quickly clogged and therefore must be constantly monitored, cleaned and/or replaced, leading to great expense and reduced efficacy over time. This problem is exacerbated by the relatively large flow rates that must be accommodated. By way of example, a filtration system may be required to process approximately 100 gallons of groundwater per minute over a 24-hour period of time, depending on the number of wells associated with the filtration system, the volume of groundwater to be lifted from each well, and the frequency at which the groundwater is lifted. Thus, approximately 144,000 gallons or 3,429 barrels of groundwater may pass through the filtration system every 24 hours.
In addition, natural gas wells are typically located at remote locations where power from electrical grids may not be available. In such locations, the wells may be operated through wind, solar or gas powered generators. Accordingly, filtration systems at remote locations should require little or no electrical power to operate.
It would therefore be desirable to provide a filtration system that is capable of removing large amounts of contaminants from groundwater under large flow rates in a relatively quick and efficient manner without substantial degradation of the filtration system. It would also be desirable to provide a filtration system that requires little or no electrical power to operate.
According to the invention, a method is provided for removing contaminants from water that has a relatively low oxygen content with naturally occurring chemoautotrophic bacteria, and at least one of iron in the ferrous state and sulfide. The method comprises oxygenating the water and directing the oxygenated water to a filtration vessel. The filtration vessel has bio-filtration media with surfaces that are exposed to the oxygenated water. The chemoautotrophic bacteria propagate in the presence of the oxygenated water and at least one of the ferrous iron and sulfide. At least one of ferric iron and sulfate are deposited on the bio-filtration media as a by-product of the chemoautotrophic bacteria. At least one form of ferric iron and iron sulfate are precipitated in the presence of the oxygenated water. The water can then be removed from the filtration vessel and either discharged into the environment or directed to a secondary filtration stage.
Further according to the invention, a system is provided for removing contaminants from water that has a relatively low oxygen content with naturally occurring chemoautotrophic bacteria, and at least one of ferrous iron and sulfide. The system comprises an oxygenation vessel for oxygenating the water and a filtration vessel having first bio-filtration media with surfaces that are exposed to the oxygenated water. With this arrangement, at least one form of ferric iron and iron sulfate can be precipitated in the presence of the oxygenated water and can be deposited on the bio-filtration media as a by-product of the chemoautotrophic bacteria to thereby remove ferric iron and/or iron sulfate from the water.